1. Field of the Invention
The invention relates to a process for producing alkylchlorosilanes from the residues from the direct synthesis of alkylchlorosilanes via heating by alternating current.
2. Description of the Related Art
When metallic silicon and alkyl chlorides RCl, where R is an alkyl moiety, are used in the direct synthesis of alkylchlorosilanes of the general formula RaHbSiCl4-a-b in which a is 1, 2, 3, or 4 and b is 0, 1, or 2, the by products comprise oligosilanes, carbosilanes, siloxanes, and high-boiling-point cracking products. The residues also comprise solids derived from the direct synthesis process which are very fine and are not retained by cyclones and filters. These solids are composed of silicon, metals, and compounds of these, metal silicides, and carbon black.
Oligosilanes make up the main part of the residues, and in particular the disilanes of the general formula RcCl6-cSi2, in which c is from 0 to 6. However, silane compounds having more than 2 Si—Si bonds can also be present, examples being trisilanes or silamethylenes. In all these formulae, silicon is tetravalent.
U.S. Pat. No. 2,598,435 describes a process in which methylchloromonosilanes are obtained from residues from the direct synthesis of methylchlorosilanes which comprise methylhalopolysilanes, at temperatures of from 250 to 800° C.
A wide variety of specifications, e.g. EP 1505070 and publications cited therein, describe processes in which residues from the direct synthesis of alkylchlorosilanes can be cleaved to give alkylchloromonosilanes, using additions of HCl and/or H2 and sometimes using complex catalyst systems. U.S. Pat. No. 2,681,355 describes a continuous process in which residues from the direct synthesis of methylchlorosilanes are reacted in an externally heated tube with HCl at temperatures of from 400 to 900° C. to form monomeric silanes, without catalysis and by a purely thermal route. Large amounts of by products are formed here, examples being carbosilanes and polymers, and also solids, because of carbonization of the residues on hot reactor walls.
DE 19711693 describes a continuous process in which alkylchlorosilanes are produced from the residues from the direct synthesis of alkylchlorosilanes, which comprise liquid constituents with a boiling point >70° C. and solids, using HCl at from 300 to 800° C. in a tubular reactor with rotatable internals. The rotatable internals are needed for scraping to remove caked material produced by carbonization on the reactor walls.